This invention relates to an adjustable radio frequency coil, and particularly to an adjustable radio frequency coil for use in a nuclear magnetic resonance imaging apparatus for medical diagnosis.
A radio frequency coil for nuclear magnetic resonance imaging is in effect a radio frequency antenna which is used to generate a radio frequency magnetic field for exciting the nuclei in a sample as well as to receive the radio frequency signals which are emitted by the excited nuclei. FIG. 1 illustrates a conventional radio frequency coil of the type described in the Journal of Magnetic Resonance, Volume 36, pages 447-451 (1979). This coil comprises a pair of plate-like H-shaped members 1 which are made of an electrically conducting material such as copper. The H-shaped members 1 are symmetrically disposed about a longitudinal axis so as to form a hollow cylinder. Each member 1 has a longitudinally-extending web 1a, which is referred to as a vertical band, and two laterally-extendings flanges 1b at opposite ends of the web 1a, the flanges 1b being referred to as wings. Adjacent flanges 1b are electrically connected with one another by means of chip capacitors 2. Two electrically conducting rings 3a and 3b are coaxially disposed within the cylinder at its opposite ends. The upper ring 3a is called a top guard ring, while the lower ring 3b is called a bottom guard ring. At least one of the two guard rings is grounded. The guard rings are separated from the inner surface of the H-shaped members 1 by a layer of an electrically-insulating material such as Teflon. The H-shaped members 1, the chip capacitors 2, and the guard rings 3a and 3b together constitute a microwave circuit which is equivalent to an LC series resonant circuit. The guard rings 3a and 3b shield a sample from the electric fields which are generated by the radio frequency voltage present between the wings and decrease dielectric losses within a sample.
The resonant frequency of the illustrated coil is dependent on the capacitance of the chip capacitors 2. However, as this coil employs fixed capacitors, the resonant frequency can be changed only by replacing the chip capacitors 2 for others with a different capacitance, making it difficult to adapt the coil to different needs and making it impossible to perform fine adjustments.